<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>mycss2</title>
    <style>
        .c1 {
            width: 400px;
            border: 1px dashed red;
        }
        .c2 {
            width: 100%;   /*大小是父元素的100%*/
            visibility: hidden;   /*隐藏*/
        }
        .c1:hover .c2 {   /*设置鼠标悬停时，内部c2的样式*/
            visibility: visible;
        }
        html {
            font-size: 16px;
        }
        .c3 {
            font-size: 2rem;  /*根元素html大小的2倍*/
        }
        .c4 {
            font-size: 2em;  /*当前大小的2倍*/
        }
        .c5 {
            width: 200px;
            height: 200px;
            /*background-color: cadetblue;*/
            /*background-color: rgb(200, 200, 200);*/
            background-color: #d7650c;
        }
    </style>
</head>
<body>
<div class="c5"></div>
<div>达内</div>
<div class="c3">
    达内
    <div class="c4">达内</div>
</div>
<div class="c1">
    <img class="c2" src="https://img02.sogoucdn.com/app/a/100520021/139c9d802f14f0c8c6f15c66ed6a9be2">
</div>
</body>
</html>